This invention relates to a solid electrolyte oxygen responsive exhaust gas sensor. It more specifically relates to enhancing response of such a sensor to oxides of nitrogen.
Automotive catalysts designed for lowering emissions of hydrocarbons, carbon monoxide and oxides of nitrogen are referred to as three-way catalysts. Emission control systems employing such catalysts have been designated as Phase II emission systems. In such systems, best results are obtained when the exhaust gas stream is maintained at stoichiometry. The exhaust gas stream is at stoichiometry when all the oxidizing and reducing species therein are chemically balanced. Hydrocarbons, carbon monoxide and hydrogen are reducing species. Oxygen and oxides of nitrogen are oxidizing species. A concentration change in any of the species will offset exhaust gas chemical balance. The three-way catalytic converter performs best at exhaust gas stoichiometry, i.e. when all oxidizing and reducing species are balanced. I recognize that this should include nitrogen oxides as an oxidizing species as well as free oxygen. For example, to maintain stoichiometry, oxygen concentration should be lowered if oxides of nitrogen increase. Otherwise, the exhaust gas will be lean, and optimum conversion will not be obtained.
Solid electrolyte exhaust gas sensors are well known for monitoring exhaust gas stoichiometry, to regulate internal combustion engine air/fuel ratio at stoichiometry. Such a sensor is frequently referred to as an oxygen sensor, since it is principally responsive to oxygen. However, I recognize that such a sensor should also be responsive to the oxides of nitrogen as well, particularly nitric oxide. Otherwise the sensor will not register a true exhaust gas stoichiometry. The sensor may be indicating stoichiometry, when the exhaust gases are in fact lean due to increased NO.sub.x content. It was not previously recognized whether such exhaust gas sensors were responsive to NO.sub.x, or whether NO.sub.x response was even significant.
The automotive exhaust gas sensor currently of greatest interest is a galvanic cell having a zirconia solid electrolyte body. The zirconia body has a reference electrode and an exhaust gas electrode. The exhaust gas electrode is a platinum film, usually with a porous overcoat to not only protect the platinum film but to enhance obtaining exhaust gas equilibrium at the platinum electrode. It has been previously proposed to make the porous overcoat of a catalytic ceramic and/or include a catalyst such as platinum in the coating to further enhance obtaining chemical equilibrium at the platinum electrode. It was expected that if chemical equilibrium were reached, true stoichiometry would be measurable. I have now found that such a sensor is not necessarily responsive to oxides of nitrogen. If so, it may be indicating stoichiometry when the exhaust gases are in fact not balanced. However, I have also found that nitrogen oxide responsiveness is obtained, without losing oxygen responsiveness, by adding rhodium to the exhaust gas electrode system. It may even be desirable to include rhodium at the exhaust electrode to insure high NO.sub.x sensitivity for longer periods of time.